Cannot call @isdefined in another macro

General Usage
1

In 0.6 I have a macro that evaluates an assignment if any of the left-hand-side variables is undefined. In Julia 0.7 I get a depwarn that urge me to change the call of isdefined to @isdefined. But @isdefined only accept a symbol rather than a variable refering to a symbol.

using MacroTools
"""@maybe a,b,c=svdfact(randn(10000,10000))"""
macro maybe(ex)
    @assert @capture(ex, ((lhs1_,lhss__)|lhs1_)=rhs_)
    lhs = [lhs1]
    lhss != nothing && append!(lhs, lhss)
    # lhs = [:v1, :v2, ...] a vector of left-hand-side symbols
    return quote
        local really = false
        for v in $lhs
            isdefined(v) || (really = true; break) # @isdefined v does not work here
        end
        really && $(esc(ex))
    end
end

In Julia 0.6, this macro only works in global scope. I only use this macro in repl/scripts so there is no problem. Now that @isdefined works for bindings in local scope, which is very nice, how should I construct the return expression of the macro?

From the implementation of @isdefined(s) I see that it simply translates to Expr(:isdefined, esc(s)). That makes :isdefined look like as special as :call, :function and :ref. I am not knowledgeable enough to understand this magic. Could you provide some pointers so that I can learn about it?

2

For now I am hand-crafting the returning Expr. But there must be some better way…right?

macro maybe(ex)
    @assert @capture(ex, ((lhs1_,lhss__)|lhs1_)=rhs_)
    lhs = [lhs1]
    lhss != nothing && append!(lhs, lhss)
    local x = Expr(:&&, Expr(:isdefined, lhs[1]), true)
    local _x = x
    for i = 2:length(lhs)
        xnew = Expr(:&&, Expr(:isdefined, lhs[i]), true)
        _x.args[2] = xnew
        _x = xnew
    end
    q = Expr(:if, x, true, ex)
    return esc(q)
end